Most gain media used in lasers have more than one possible lasing transition. When excited, the laser will generate one or more wavelengths of light. The predominant wavelength or the wavelength with the greatest power is typically the wavelength with the highest gain. It is frequently desired to configure the laser to oscillate primarily or solely at a wavelength other than the predominant wavelength and which has a significantly lower gain.
A number of approaches have been developed in the prior art to suppress high gain wavelengths in favor of lower gain wavelengths. One common approach is to use wavelength selective coatings. For example, the coating on a resonator mirror can be highly reflective for the low gain wavelength and transmissive for the high gain wavelength.
Another approach is to use a dispersing prism which varies the path of the beam with respect to its wavelength. The prism can be placed intracavity or used as an end mirror in a Littrow configuration. In either case, by adjusting the angle of the prism, the laser can be forced to lase at the desired wavelength. Still another approach is to rely on intracavity filters such a etalons or birefringent tuning elements.
All of the above techniques work well in most applications. However, the techniques are not completely successful for selecting a very low gain wavelength when the gain medium has a comparatively very high gain wavelength. An example of this situation is in Nd:YAG which has a very high gain wavelength at 1.06 microns, a medium gain wavelength at 1.33 microns and very low gain wavelengths at 0.94, 1.44 and 1.83 microns.
The following chart found in IEEE Quantum Electronics, Volume QE14, No. 1, January, 1978, an article by Marking, illustrates a measure of the relative gains in some of the Wavelengths in Nd:YAG, under continuous pumping conditions.
______________________________________ Main room-temperature transitions in ND:YAG Wavelength Relative ([.mu.m], air) Transition Performance ______________________________________ 1.05205 R.sub.2 .fwdarw. Y.sub.1 46 1.06152 R.sub.1 .fwdarw. Y.sub.1 92 1.06414 R.sub.2 .fwdarw. Y.sub.3 100 1.0646 R.sub.1 .fwdarw. Y.sub.2 .about.50 1.0738 R.sub.1 .fwdarw. Y.sub.3 65 1.0780 R.sub.1 .fwdarw. Y.sub.4 34 1.1054 R.sub.2 .fwdarw. Y.sub.5 9 1.1121 R.sub.2 .fwdarw. Y.sub.6 49 1.1159 R.sub.1 .fwdarw. Y.sub.5 46 1.12267 R.sub.1 .fwdarw. Y.sub.6 40 1.3188 R.sub.2 .fwdarw. X.sub.1 34 1.3200 R.sub.2 .fwdarw. X.sub.2 9 1.3338 R.sub.1 .fwdarw. X.sub.1 13 1.3350 R.sub.1 .fwdarw. X.sub.2 15 1.3382 R.sub.2 .fwdarw. X.sub.3 24 1.3410 R.sub.2 .fwdarw. X.sub.4 9 1.3564 R.sub.1 .fwdarw. X.sub.4 14 1.4140 R.sub.2 .fwdarw. X.sub.6 1 1.4440 R.sub.1 .fwdarw. X.sub.7 0.2 ______________________________________
In the situation where the relative gains are so different, a very low level of optical feedback of a high gain wavelength will cause the laser to oscillate at that wavelength. The energy taken by the high gain wavelength will rob the low gain wavelength of substantial power, even to the extent of extinguishing it.
Since the discrimination techniques discussed above are not perfect, some feedback of the high gain wavelength is usually encountered which substantially degrades the output of the low gain wavelength. More particularly, there are no single surface coatings available which can provide the virtually absolute discrimination necessary to suppress the high gain 1.06 micron wavelength in Nd:YAG, especially in the case of pulsed pumping at high inputs to the flashlamps. In addition, in the near infrared, most common glasses used to form intracavity prisms have insufficient dispersion characteristics to adequately discriminate against the 1.06 micron wavelength. Accordingly, in the prior art, in order to maximize power in a low gain wavelength, it was necessary to use a combination of techniques to suppress the high gain wavelength.
Therefore, it would be desirable to provide a simple approach for a two mirror cavity resonator which would suppress a high gain wavelength and allow a low gain wavelength to lase.
Accordingly, it is an object of the subject invention to provide a resonator mirror configured to suppress a high gain wavelength while allowing a lower gain wavelength to lase.
It is another object of the subject invention to provide a resonator mirror which includes coatings on two surfaces for suppressing the oscillation of a high gain laser wavelength.
It is a further object of the subject invention to provide a resonator mirror which has a wedge shaped configuration and is oriented to reflect the high gain wavelength out of the cavity while reflecting the low gain wavelength back along the resonator axis.
It is still another object of the subject invention to provide a resonator structure for suppressing the 1.06 wavelength emission from Nd:YAG while allowing wavelengths of lesser gain to oscillate.